Concrete has been used extensively in a wide variety of applications for many years. It is particularly employed as a basic raw material by the civil engineer. Its relatively low cost and high compressive strength, and the ease of pouring of the freshly prepared, water-mixture into forms of desired shapes are advantages which adapt it for such uses as buildings, industrial silos, highways, bridges and dams. More recently, its use in nuclear power plants has become common practice.
Special types of concrete have been developed with particularly desirable properties to meet the requirements of special situations. Low density concrete may be cited as an example of a modified formulation designed to provide the characteristics of light weight and low thermal conductivity which are desired in building construction. Other examples include special formulations involving use of certain additives to improve water tightness, and other additives to increase air entrainment to improve workability of freshly prepared wet mixtures before pouring or pumping.
Earlier, it was noted that a product made from an iron ore concentrate possessed special characteristics, which were particularly useful for the attenuation of nuclear radiation. Disclosure of this fact was made in U.S. Pat. No. 3,645,916 issued Feb. 29, 1972.
Concrete formulations have used many different types of aggregates, usually with a wide range of sizes, often including mesh sizes of two or three inches or more. Use of natural rock or ore of large size often leads to a structure which is non-uniform in composition, because the aggregate size distribution of the rock or ore as it comes from the gravel pit or mine may not be easily reproducible from one delivery to the next. Large size aggregates in the concrete mix tend to bridge during pouring to form air pockets, especially around reinforcement bars and in corners of forms. Large aggregates also often are caught and held in baffles in mixing equipment, which may cause breakage of this equipment, with the resultant delays of operations. It also is very difficult to pump mixtures containing large aggregates, and thus this efficient method of construction is often excluded in these situations.
Aggregates in concrete produce differences in composition within the body of the set material. Often there is a significant differential in certain physical properties of the different compositions, such as thermal conductivity and thermal expansion. With temperature and other changes, these differentials create unequal stresses, leading to crack-generating stress peaks because of deformation incompatability.
Properties of Concrete (A. M. Neville, Pp. 226), referring to concrete, states: "The theoretical strength has been estimated to be as high as 1.5.times.10.sup.6 lb/inch.sup.2. This discrepancy can be explained by the presence of flaws postulated by Griffith. These flaws lead to high stress concentrations in the material under load so that a very high stress is reached in very small volumes of the specimen with a consequent microscopic fracture, while the average (nominal) stress in the whole specimen is comparatively low. The flaws vary in size and it is only the few largest ones that cause failure; the strength of a specimen is thus a problem of statistical probability."
The mean free path for a fracture in the matrix of a mortar is less than in the matrix of a concrete. Also the length of a separation between the matrix and the miniaggregate in a mortar is much less than the possible length of a separation between the matrix and the ever-so-much larger aggregate of ordinary concrete. Furthermore, average aggregates of concretes, including natural ores used for densified concretes, are large in size and differ in composition within individual pieces. In concretes, conventional or densified, probability of cracking at grain boundaries between the two phases is much greater than in the material with more homogeneous miniaggregate. This is attributed to the fact that a mini-aggregate has few, if any, grain boundaries within itself.
Concrete is much stronger in compression than in tension, and this characteristic has led to the use of a technique known as "prestressing". In this process, the material is placed initially during fabrication under considerable stress in a manner that will oppose any load applied to the structure afterwards. Presently, there is considerable interest in using the prestressed method in the nuclear industry in connection with the construction of prestressed concrete reactor vessels, commonly called PCRV, as an alternate to the steel reactor pressure vessel.